Title: OPTICAL%20ISOMERISM
1OPTICAL ISOMERISM
R W Grime Ripon Grammar School
Molecular mirror images
Optical isomerism and chirality
Enantiomers and racemates
Self-Test recognising optical isomers from
structural formulae
Polarimetry recognising optical isomers by
experiment
Examples of optical isomers and their importance
thalidomide
carvone
limonene
2- All molecules have a mirror image but for most
molecules it is the same molecule.
3- For some molecules the mirror image is a
different molecule (the mirror image is
non-superimposable).
4- Left and right hands are an example of
non-superimposable mirror images.
5- This usually happens when a molecule contains a C
atom with four different groups attached (chiral
/ asymmetric C). - Such molecules are said to be chiral or optically
active.
6- The optical isomers are called enantiomers.
- These are distinguished by /-, D/L or more
correctly R/S. - A 50/50 mixture of the two enantiomers is called
a racemic mixture or a racemate and shows no
optical activity
7- TASK Some of the following molecules are
optically active. For each one, click its name
below and decide whether it is optically active
or not. Click again to see if you are correct.
a) propan-2-ol
e) butanone
b) 2-chlorobutane
f) 2-methylbutanoic acid
c) 1-chlorobutane
g) butan-2-ol
d) 3-methylhexane
h) 1-chloro-3-methylpentane
8NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
9Click here to go back to the optical isomerism
task
10NOT OPTICALLY ACTIVE
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task
11Click here to go back to the optical isomerism
task
12NOT OPTICALLY ACTIVE
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task
13NOT OPTICALLY ACTIVE
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task
14Click here to go back to the optical isomerism
task
15Click here to go back to the optical isomerism
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16Click here to go back to the optical isomerism
task
17- Molecules that are optical isomers are called
enantiomers.
- Enantiomers have identical chemical and physical
properties, except
- Their effect on plane polarised light
- Their reaction with other chiral molecules
18- Light is a form of electromagnetic radiation.
19- The wave vibrations are perpendicular to the
direction of travel of the wave.
20- Optical isomers rotate the plane of plane
polarised light.
21- POLARIMETERS can be used to analyse the effect
optical isomers have on plane polarised light
Heriot Watt University has a web page with an
interactive tutorial and self-test questions
about this topic http//scholar.hw.ac.uk/site/che
mistry/activity5.asp?outline
22- Chiral molecules often react differently with
other chiral molecules. - This is like the idea that a right hand does not
fit a left handed glove the molecule must be
the correct shape to fit the molecule it is
reacting with. - Many natural molecules are chiral and most
natural reactions are affected by optical
isomerism.
23- For example, most amino acids (and so proteins)
are chiral, along with many other molecules. - In nature, only one enantiomer usually occurs
(e.g. all natural amino acids rotate polarised
light to the left).
24- Many drugs are optically active, with one
enantiomer only having the beneficial effect. - In the case of some drugs, the other enantiomer
can even be harmful, e.g. thalidomide.
25- In the 1960s thalidomide was given to pregnant
women to reduce the effects of morning sickness. - This led to many disabilities in babies and early
deaths in many cases.
The photographs are both from Molecule of the
Month at Bristol University http//www.chm.bris
.ac.uk/motm/thalidomide/start.html
26S thalidomide (effective drug) The body
racemises each enantiomer, so even pure S is
dangerous as it converts to R in the body.
R thalidomide (dangerous drug)
27- Thalidomide was banned worldwide when the effects
were discovered.
28Your nose is chiral too!
29S carvone (caraway seed)
R carvone (spearmint)
Caraway Seed has a warm, pungent, slightly bitter
flavour with aniseed overtones.
30S limonene (lemons)
R limonene (oranges)